Significantly improved peak rates of 1 Gbps in the downlink and 500 Mbps in the uplink are required for a Long Term Evolution-Advanced (LTE-A) system as compared to a Long Term Evolution (LTE) system. Also good compatibility of the LTE-A system with the LTE system is required. The technology of Carrier Aggregation (CA) is introduced to the LTE-A system to accommodate the required improved peak rates, compatibility with the LTE system and full use of spectrum resources.
The CA technology refers to that a user equipment can simultaneously work on a plurality of cells, and one cell includes a pair of uplink (UL)/downlink (DL) Component Carriers (CCs). The LTE system or any of previous wireless communication systems includes only one pair of CCs. CCs in a carrier aggregation system may be continuous or discontinuous, the bandwidths between the CCs may be the same or different, and to keep compatible with the LTE system, the maximum bandwidth of each CC is limited to 20 MHz. At present, it is generally recognized that the number of cells which may be aggregated by the User Equipment (UE) is 5 at most. Moreover, the carrier aggregated cells are classified in the LTE-A system, namely divided into:
a Primary Cell (PCell): only one cell in the cells aggregated by the UE is defined as the PCell;
Secondary Cells (SCells): all other cells aggregated by the UE except the PCell are referred to as the SCells.
The PCell is selected by a Node B and configured to the user equipment through Radio Resource Control (RRC) signaling, and the PCell of different user equipments may be different. No matter the PCell or the SCells, each cell has an independent Hybrid-ARQ (HARQ) entity which maintains a series of independent processes.
For saving power, a Discontinuous Reception (DRX) mechanism is introduced into the LTE system. Under the DRX mechanism, monitoring by the UE on a Physical Downlink Control Channel (PDCCH) is controlled through timers, and the relevant timers are described as follows.
OnDurationTimer (continuous monitoring timer): the UE periodically wakes and monitors the time of the control channel. The length of the timer is configured through RRC signaling, and has a minimum psf 1 and a maximum psf 200 by taking a PDCCH subframe as a unit (psf). The so-called PDCCH subframe refers to a subframe with the PDCCH.
DrxShortCycleTimer (discontinuous reception short cycle timer): to better match the characteristic of data service arrival, configuration of two DRX cycles (discontinuous reception cycle), namely a long cycle and a short cycle, is allowed in the LTE system. The two cycles has the same onDurationTimer, but has different sleep time. In the short cycle, the sleep time is shorter, and the UE may monitor the control channel again more quickly. The long cycle must be configured, and is the initial state of the DRX process: and the short cycle is optional. A duration adopting the short cycle is set in the DrxShortCycleTimer, and after the DrxShortCycleTimer is overtime, the UE uses the Long cycle. The DrxShortCycleTimer is configured through RRC signaling, and has a length taking the number of the short cycles as a unit, which ranges from 1 to 16.
Drx-InactivityTimer (discontinuous reception-inactivity timer): after the DRX is configured, when the UE receives control signaling of initial transmission of an HARQ within an Active Time allowing monitoring of the control channel, the timer is started, and before the timer is overtime, the UE continuously monitors the control channel. If the UE receives the control signaling of the initial transmission of the HARQ before the Drx-InactivityTimer is overtime, the Drx-InactivityTimer is terminated and restarted. The length of the timer is configured through RRC signaling, and has a minimum psf 1 and a maximum psf 2560 by taking the PDCCH subframe as a unit (psf).
HARQ RTT Timer (hybrid automatic repeat request round-trip timer): the timer is merely applicable to a Downlink (DL), so that the UE may not monitor the control channel before next retransmission arrives, and a better power saving effect is achieved. If the UE receives scheduling signaling of HARQ transmission (initial transmission or retransmission), the timer is started. If data in the corresponding HARQ process are not decoded successfully after last HARQ transmission, namely the UE feeds back NACK (non-acknowledge) information, after the HARQ RTT Timer is overtime, the UE starts a Drx-RetransmissionTimer. If data in the corresponding HARQ process are decoded successfully after last HARQ transmission, namely the UE feeds back ACK (acknowledge) information, after the HARQ RTT Timer is overtime, the UE does not start the Drx-RetransmissionTimer. If only the HARQ RTT Timer runs currently, the UE does not monitor the control channel.
Drx-RetransmissionTimer (discontinuous reception-retransmission timer): the timer is merely applicable to DL. During operation of the Drx-RetransmissionTimer, the UE monitors control signaling, and waits for retransmission scheduling of the corresponding HARQ process. The length of the timer is configured through RRC, and has a minimum psf 1 and a maximum psf 33 by taking the PDCCH subframe as a unit (psf).
From the above-mentioned descriptions, the lengths of the onDurationTimer, the drx-InactivityTimer and the drx-RetransmissionTimer in the timers related to the DRX are counted based on the number of PDCCH subframes. For TDD, the PDCCH subframe refers to a downlink subframe including a Downlink Pilot Time Slot (DwPTS) in a special subframe.
The LTE-A system still uses the DRX mechanism of the LTE system. Just because the LTE-A system adopts the carrier aggregation technology, for how to use the DRX under multiple carriers, the present method is to adopt a common DRX mechanism, namely all cells have the same Active Time.
Random access methods in the LTE system and the LTE-A system in the prior art will be introduced below.
The random access objective of the LTE system is mainly to establish RRC connection or uplink synchronization. At present, two random access schemes are supported, i.e., competitive random access and noncompetitive random access.
The process of the noncompetitive random access is shown in FIG. 1, and mainly includes three steps:
Msg0: an Evolved Node B (eNB) allocates a dedicated Random Access Preamble (ra-Preamblelndex) for the noncompetitive random access and a Physical Random Access Channel (PRACH) Mask Index (ra-PRACH-MaskIndex) for the random access to UE, wherein the information is carried by a PDCCH order for the noncompetitive random access caused by downlink data arrival and carried by a handover command for the noncompetitive random access caused by handover;
Msg1: the UE sends a designated dedicated preamble to the eNB on a designated PRACH resource according to the ra-Preamblelndex and ra-PRACH-MaskIndex indicated in the Msg0, and the eNB receives the Msg1 and then calculates an uplink timing advance according to the Msg1;
Msg2: the eNB sends a random access response including timing advance information to the UE, to inform the UE of the timing advance of follow-up uplink transmission.
The competitive random access flow is shown in FIG. 2, and mainly includes four steps:
Msg1: UE selects a random access preamble and a PRACH resource, and sends the selected random access preamble to an eNB using the PRACH resource;
Msg2: the eNB receives the preamble, calculates a timing advance (TA) and sends a random access response to the UE, wherein the random access response at least includes the timing advance information and an uplink scheduling grant (UL grant) for Msg3;
Msg3: the UE transmits uplink transmission on the UL grant designated in the Msg2, and for different random access reasons, the contents of the uplink transmission in the Msg3 are different, for example, for an initial access, an RRC connection establishment request is transmitted in the Msg3;
Msg4: a competition solution message, the UE may judge whether the random access is successful according to the Msg 4.
The carrier aggregation technology is introduced into the LTE-A system. An R10 version does not support Multi-TA, namely all the cells aggregated by the UE use the same uplink timing advance as the PCell, so the random access merely occurs in the PCell in R10. Different from the R10 version, an LTE-A R11 version supports multi-TA, namely different cells aggregated by the UE use different uplink timing advances. To facilitate maintenance of the multi-TA, a concept of uplink timing advance group (TA group, TAG) is introduced, all cells in one TAG use the same uplink timing advance, wherein the TAG including the PCell is referred to as a PTAG, and the TAGs merely including the SCells are referred to as STAGs. For each TAG, the uplink timing advance needs to be acquired through a random access process. For the PTAG, the acquisition of the uplink timing advance may adopt competitive or noncompetitive random access; and for the STAGs, the uplink timing advance is generally acquired through the noncompetitive random access.
About L2 measurement in the LTE system:
in the LTE system, for the objective of load balance or network performance monitoring by an Operation and Management System (OAM), the protocol defines a series of layer 2 (L2) measurements, including Physical Resource Block (PRB) usage, number of activated UE, packet delay, packet loss rate, scheduling throughput and the like, and the eNB acquires these measurements and then reports the measurement results to the OAM, so that the OAM may master the network performance and adjust and optimize network configurations. Wherein, the PRB usage is measured in two manners:
Total PRB usage: counted in Up-Link (UL) and DL respectively, namely a ratio of the sum of UL/DL PRBs for transmission to the sum of UL/DL available PRBs:
PRB usage per traffic class: counted in UL and DL respectively according to QCIs (Quality of service Class Identifier), namely a ratio of the sum of UL/DL PRBs occupied by a Dedicated Transport Channel (DTCH) for transmitting each QCI class to the sum of UL/DL available PRBs.
About Time Division Duplex (TDD) ULDL configurations:
in R8, R9 or R10, a physical layer standard defines the following seven UL/DL configurations for the TDD system, shown in the following table, wherein D represents a DL subframe. U represents a UL subframe, and S represents a special subframe of the TDD system.
TABLE 1TDD UL/DL ConfigurationsUL/DLSubframe NumberConfiguration01234567890DSUUUDSUUU1DSUUDDSUUD2DSUDDDSUDD3DSUUUDDDDD4DSUUDDDDDD5DSUDDDDDDD6DSUUUDSUUD
In an R11 system, the CA user equipment of LTE-A may share or use adjacent bands with other systems, such as the LTE system. As shown in FIG. 3, the LTE-A user equipment aggregates three cells: Cell 1, Cell 2 and Cell 3. The Cell 1 and the Cell 2 use the same band (Band 1), whereas the Cell 3 uses Band 2. To avoid uplink/downlink cross interference of the TDD system, the Band 1 and a 3G/LTE TDD Band A should use compossible TDD UL/DL configurations, and the so-called compossible TDD configurations refers to configurations without UL/DL cross interference and refers to the same TDD UL/DL configuration for the LTE system. The Band 2 and a 3G/LTE TDD Band B should use compossible TDD UL/DL configurations. If the TDD ULDL configurations used by the Band A and the Band B are different, the TDD UL/DL configurations used by the Band 1 and the Band 2 are different either.
For the carrier aggregation system, if a plurality of cells aggregated by the user equipment have different TDD ULDL configurations, uplink/downlink collision subframes may be produced, such as subframe #3, subframe #4, subframe #8 and subframe #9 shown in FIG. 4. For processing of the uplink/downlink collision subframes at present, a definite stipulation has not been given in the prior art.